The present invention generally relates to radio frequency receivers and more particularly to RF receivers that incorporate a superregenerative oscillator in their design.
Superregenerative receivers have been known for some time and are widely used in garage door openers and automotive remote entry systems. These applications require a low cost receiver with simple manufacturing processes, high reliability, and moderate performance specifications. At the present time, typical operating distance specifications for automotive remote entry systems is about 10 meters. In addition, vehicle remote entry receivers must operate when the ignition key is in the OFF position, and therefore must operate with low current, typically about 1 milliamp (mA). The garage door opener and automotive remote entry systems typically use On-Off Keying (OOK) modulation. In this type of modulation, the carrier is present during one logic level, and is not present during the other logic level as is shown in the plot illustrated in FIG. 8.
Superregenerative receivers can offer several advantages over other types of receiver, such as homodyne or superheterodyne technologies. Superregenerative based receivers are low cost, have few components, require low current, and provide acceptable range performance under most conditions. The disadvantages of such receivers can include high emissions over a broad frequency range if the layout is not correct or if it does not include a high reverse isolation preamplifier, poor selectivity if input filtering is inadequate, and a tendency to react to frequencies that are sub-harmonic of the desired operating frequency. For example, a 315 MHz superregenerative oscillator will react to the ⅓ sub-harmonic of 105 MHz. This is problematic in the United States because 315 MHz is an automotive remote entry and garage door frequency and 105.1 MHz is a widely used FM radio frequency. It is therefore necessary to reject sub-harmonic frequencies to assure proper operation and such rejection is achieved with input filtering.
Another problem that is becoming more common in automotive remote entry receivers is inadequate range performance, or equivalently, inadequate sensitivity. This can be caused in any system by sub-optimal antenna placement within the vehicle. In addition, several vehicle manufacturers are increasing their range performance specifications. For this reason, many receiver designs now employ superheterodyne circuits to address this issue. Superheterodyne receivers have significantly higher costs and consume higher currents than do superregenerative solutions. Thus, in addition to higher cost, superheterodyne circuits require current reduction schemes that increase overall circuit complexity.